If you get a message saying that there is a security hole in Steam exploited by NisseFar or Nivek telling you to pass it on, don't. It's a prank. It started out as a small joke on NisseFar and Nivek done by their friends, but some friend of a friend brought it back and it's become quite infamous. It's made a comeback recently, so beware. If there's one time when it's better sorry than safe, this is it. I learned that the hard way.

On the other hand, there are some fake trade deals instructing you to download a program (hint, it's a virus) so watch out for that. It's pretty easy to notice, but I'll repeat it just in case you aren't familiar with the fake download link scam.